Aqueous acidic chromium passivating solutions are in widespread commercial use for improving the corrosion resistance and for enhancing the appearance of various receptive metal surfaces including zinc, zinc alloy, cadmium, cadmium alloy surfaces and the like. Such passivating solutions can be controlled to impart a yellow or a blue-bright coating to the surface in which the latter simulates a chromium finish.
Such chromium passivate solutions conventionally contained chromium ions in the hexavalent state but in more recent years have been replaced by trivalent chromium ions in view of the increased simplicity and efficiency in treating waste effluents containing chromium in the trivalent state. While initially, trivalent chromium passivate solutions were less effective than the conventional hexavalent chromium passivating compositions, further research and development has now produced trivalent chromium passivate compositions which provide for acceptable passivate films on a variety of receptive metal substrates. Typical of such processes are those described in U.S. Pat. Nos. 4,359,345; 4,359,346; 4,359,347; 4,359,348; 4,349,392; and 4,367,099 which are all assigned to the assignee of the present invention and the teachings of which are incorporated herein by reference to the extent that the disclosures contained therein are relevant to the subject matter of the present invention. Typical other prior art compositions and processes for treating zinc and zinc alloy surfaces are those such as disclosed in U.S. Pat. Nos. 2,393,663; 2,559,878; 3,090,710; 3,553,034; 3,755,018; 3,795,549; 3,843,430; 3,880,772; 3,932,198; 4,126,490; 4,171,231; 4,263,059; British Pat. No. 586,517 and No. 1,461,244; and German Pat. No. 2,526,832.
Chromium passivate compositions of the types commercially employed and as described in the aforementioned patents invariably employ peroxide-type oxidizing agents, particularly hydrogen peroxide, which is a necessary bath constituent to produce acceptable passivate films. A continuing problem associated with such peroxide-type passivating compositions is the high rate of consumption and relatively rapid loss of the peroxide oxidizing agent necessitating frequent replenishment and a further careful control of pH by adjustment due to the relatively rapid rise in pH. The consumption of peroxide is due in part to the presence of various activating metal ions present in the solution as well as contaminating metal ions such as zinc, cadmium and iron, for example, which are introduced by the dissolution of the metal from the substrates being treated which tend to catalyze a decomposition of the oxidizing agent. The frequent replenishment of the hydrogen peroxide is not only time consuming but is also relatively costly. Such depletion of the peroxide oxidizing agent also occurs when the bath stands overnight and over weekends between operating shifts.
The advent of high-speed treatment of metal surfaces such as zinc, for example, in which the aqueous acidic chromium passivating solution is spray applied to the metal surface for short periods of time such as 1 to 10 seconds has further aggravated the problem due to the increased loss of the peroxide constituent in such spray applications and the volatilization and contamination of the surrounding atmosphere providing an environmentally objectionable working condition. High-speed immersion type applications in which jets of the passivate solution are directed against the surface to be treated also result in an excessive loss of the peroxide-type oxidizing agent.
It has also been observed in the processing of work pieces incorporating apertures which have not received a coating or plating of metal such zinc, for example, of which metal tubing is typical, an immersion of such tubing into a peroxide-type chromium passivate solution causes a rapid flash-rusting of the interior of the tubing in a matter of only several seconds causing rust spots and further causing an excessive consumption of the peroxide oxidizing agent during its reaction with the exposed ferrous metal on the interior of the tubing.
The present invention overcomes many of the disadvantages and problems associated with prior art trivalent-type chromium passivating solutions employing an alternative non-peroxide oxidizing agent which is of relatively low volatility, is inexpensive, does not form environmentally objectionable vapors, can be employed in high-speed passivating processes without objectionable loss of the oxidizing agent, can be employed for passivating work pieces having exposed bare iron or steel surfaces such as tubing wherein the solution itself serves to passivate rather than flash oxidize such bare surfaces, and wherein the composition and the process of use of the composition is of simple and economical operation and control.